This invention relates to oxybenzoyl copolyester fibers of increased strengths and to methods for the manufacture thereof. More particularly, it relates to essentially linear copolyesters, in fiber form, made from oxybenzoyl, phthaloyl and bisphenoxy moieties which are increased in tensile strength and Young's modulus and are diminished in elongation by heating and stretching. Products made are of strengths equivalent or superior to those of the best of all other synthetic organic polymers known.
Para-oxybenzoyl homopolyesters have been described in U.S. Pat. Nos. 2,600,376; 2,728,747; and 3,039,994 and in an article by Gilkey et al, entitled Polyesters of Hydroxybenzoic Acids, appearing in the Journal of Applied Polymer Science, Vol. II, No. 5, pages 198-202 (1959). Improved p-oxybenzoyl homopolyesters have more recently been the subject of French Pat. No. 1,568,152, and applications of such materials and methods for the manufacture thereof have been described in U.S. Pat. Nos. 3,644,593; 3,656,994; 3,662,052; 3,668,300; and 3,669,738. Because of some processing difficulties encountered with the oxybenzoyl homopolyesters, certain copolyesters including oxybenzoyl linkages have been made which are of improved processing properties. For example, the polymers which are described in U.S. Pat. No. 3,637,595 are more easily molded than those of the French patent and are useful for forming into various shapes by known techniques. Thus, they may be employed for producing molded articles, coatings, and fibers and such are described in the patent.